1. Field of the Invention
The present invention relates to a method of manufacturing an optical semiconductor device that uses an optical semiconductor chip such as an LED lamp. More particularly, an embodiment of the optical semiconductor device is capable of preventing a semiconductor chip to be damaged from variations in environmental temperature, thus improving reliability and preventing deterioration in performance due to the improved reliability, thus allowing reliability and performance to exist together.
2. Description of the Related Art
When an improvement of reliability is intended in a conventional optical semiconductor device, an optical semiconductor chip is sealed in a first, relatively soft resinous member to improve resistances against shock and moisture. Further, the first resinous member is sealed in a second, relatively hard resinous member to improve the mechanical strength. In this way, two resins with different characteristics can be employed together to compensate for a function lacked in one from another and to improve the reliability. (See Patent Publication 1: JP-A-05/327029, paragraphs 5-7, FIG. 1, for example).
In the above conventional structure, however, when selecting the two resins, the relatively soft resin is commonly selected as the inner resin that directly contacts the optical semiconductor chip and the relatively hard resin as the outer resin to achieve the required mechanical strength.
In such a case, there is a difference in the thermal expansion coefficient between the soft resin and the hard resin. The difference may reach a factor of 10 times, depending on the case. Under a high-temperature ambient, the soft resin that is surrounded in the hard resin expands in volume and applies an excessive compressive pressure to the optical semiconductor ship. Therefore, a problem is caused because such pressure can be a factor in deterioration of the chip.
To avoid this and other problems, in a proposed optical semiconductor device 90 shown in FIG. 5, a space B can be provided between a soft resin 91 and a hard resin 92. This space is effective to prevent a compressive stress from being imposed on an optical semiconductor 93 even if a volume expansion occurs in the soft resin 91 as an environmental temperature elevates.
In the above example, the light emitted from (or coming into) the optical semiconductor chip 93 passes through the space B. That is, the light passes through air, which has a lower refractive index compared to the resin. In this case, reflection can occur at an interface between the resin, and the air can cause a loss in amount of light, resulting in an output reduction of about 25-35%. Such an output reduction causes a problem associated with deterioration of the performance of the optical semiconductor device.
As for the soft resin 91 and the holder 94, a difference in expansion coefficient between respective materials can also cause a problem because it may form peeled or cracked parts in the product.
The holder 94 commonly employs a lead-frame-insert injection-molded product in its structure. In this molding method, an air layer can be interposed between the frame and the insert-molded product. The air layer expands under a high-temperature ambient and applies pressure to the soft resin, which produces bubbles therein. The production of bubbles causes a problem because it lowers the optical output similar to the above case and deteriorates the performance of the optical semiconductor device.